Generally, when a battery for the cellular phone handset or the like is charged, there is provided a constant current constant voltage charging system. This is a charging circuit that charges the battery to a certain battery voltage by constant current charging and then makes transition to constant voltage charging, when the battery is charged. When the charging is performed by a commercially available charger having a low charging current supply capability other than a normal charger in this charging circuit, charging of the battery is performed with a certain large current in consideration of a battery charging completion time, which is one of performance of the cellular telephone handset. Further, when transition to the constant current charging is made, the charging is instantaneously started from precharging, using a certain large current. For this reason, when the transition to the constant current charging is made, a supply voltage on a charger side is reduced to below a chargeable voltage. The charging is thereby stopped. Then, the supply voltage on the charger side is increased due to stop of the constant current charging, whereafter the constant current charging is started again. Start/stop of the charging as described above is thus repeated, so that the battery is not charged.
As conventional art 1, Patent Document 1 (JP Patent Kokai Publication No. JP-A-10-51970) describes a charging system that gradually increases a charging current at a time of trial charging (precharging) (in FIG. 1 in the publication). This charging system is shown in a block diagram in FIG. 5 of this application. In this charging system, a case where a charger 20 is connected to an external power supply 30 of an AC power supply via a connector 4a and a battery 1a is connected to this charger 20 is shown. When the battery 1a is charged using this charger 20, a charging control device 21 controls a charging current IB supplied from the charger 20 to the battery Ia. The charger 20 converts AC power supplied from the external power source 30 into direct current under control of the charging control device 21 and supplies the direct current to the battery Ia.
The charging control device 21 receives a terminal voltage VB of the battery 1a from a voltage sensor 22 annexed to the battery 1a and a temperature T of the battery 1a from a temperature sensor 23 annexed to the battery 1a. Then, according to these voltage VB and temperature T, the charging control device 21 controls the charging current IB of the charger 20. A limiter 24 that is used together with the charging control device 21 supplies a command to the charging control device 21 when the terminal voltage VB detected by the voltage sensor 22 has increased to a predetermined limiter operating voltage VLIM, thereby stopping charging of the battery Ia by the charger 20. The limiter 24 thereby prevents overvoltage of the battery 1a. 
The charging control device 21 first determines whether or not the temperature T obtained by the temperature sensor 23 is above a charging default temperature T0, at a time of starting charging. In this conventional art 1, there is no problem when T>T0 holds (when the temperature of the battery 1a is higher). When T>T0 does not hold (when the temperature of the battery Ia is lower), the terminal voltage VB may increase in a comparatively rapid manner with progress of the charging. Accordingly, the charging control device 21 exercises control so that trial charging is performed.
When T>T0 holds, the charging control device 21 exercises control so that a value of the charging current IB assumes a predetermined current value Ik. Herein, it is set that the more a number k increases, the more the current value Ik is reduced. The charging control device 21 maintains this control state until VB≧VBT holds as a result of the terminal voltage VB having increased with the progress of charging. When VB≧VBT is established, the charging control device 21 executes a predetermined charging completion process. When T>T0 does not hold, i.e., when it is determined that the temperature T of the battery Ia is comparatively low, the terminal voltage VB may increase in the comparatively rapid manner with the progress of charging. Thus, the charging control device 21 exercises control so that the trial charging is performed.
When the charging control device 21 exercises control so that the trial charging is performed, the charging control device 21 sequentially increment an order number j, determines a duration tj based on correspondence between the temperature T and the duration tj, and exercises control so that the charging current IB becomes current ij. Herein, the duration tj is a duration in a stage where the battery 1a is charged with a constant current ij. The correspondence between the duration tj and the temperature T is set so that the more the temperature T is increased, the more the duration tj is reduced. It is further set that the more the number j is increased, the more the duration tj is increased. The charging control device 21 maintains this control state until VB≧VBT or t≧tj holds. When t≧tj holds, the charging control device 21 sequentially increases the number j one by one. Then, when VB≧VBT holds, the charging control device 21 determines whether or not there is the current Ik which satisfies Ik<ij<Ik-1. When such Ik is present, the charging control device 21 exercises control so that a value of the charging current IB assumes the predetermined current value Ik. When such the current Ik is not present, i.e., when ij is smaller than a current value In, for example, the charging control device 21 finishes operation.
Patent Document 1: JP Patent Kokai Publication No. JP-A-10-51970